Many plastic materials derived from petroleum have considerably various applications from daily necessaries to aero-space materials, as the mechanical properties and thermal stability thereof can be easily controlled. However, when plastic wastes are buried in a landfill, these are not decomposed but accumulated in the environment. In the meantime, when such plastic wastes are incinerated, harmful by-products and a great amount of carbon dioxide are generated, thereby promoting environmental contamination as well as global warming.
With an increasing concern on the environmental problems, researches on eco-friendly plastics such as plastics made of plant resources or biodegradable plastics which are decomposed by microorganisms have been increasingly carried out. As for the eco-friendly plastics under consideration, there are polyhydroxy butylate, polycaprolactone, polylactic acid, aliphatic polyesters, biopolyethylene, etc., and among them, polylactic acid has been most widely and aggressively investigated and appreciated for having an excellent availability in a commercial point of view.
The polylactic acid is a relatively rigid polymer with a flexural modulus as much as that of general plastics, but poor heat resistance and molding properties. Owing to such defects, it has limited applications where require high heat resistance. In this respect, studies for complementing the properties of polylactic acid by mixing it with petroleum-derived plastics such as polypropylene or polycarbonate, etc. have been being made, however in most of such cases, the polylactic acid content cannot be over 50 weight percent (wt %) and thus it is not suitably referred as genuine eco-friendly plastic materials.
For ensuring high heat resistance and molding properties of the polylactic acid, there is a method for improving the crystallinity degree of the polymer. Currently, for improving the crystallinity degree of the polylactic acid, it is general to increase the temperature of a mold for injection molding and to lengthen the cooling time in the mold, however this method has a disadvantage which is a prolonged molding cycle. For reducing the cooling time, a method for promoting crystallization rate (hereinafter, also referred as a crystallinity degree) by adding a seeding agent for crystallization (i.e. nucleating agent) is known in the art. A nucleating agent promotes the growth of a crystal by being served as a primary crystal nucleus of a crystalline polymer, makes the crystal size finer, and increases the crystallization rate. As for a nucleating agent used for a polylactic acid resin, inorganic particles consisting of talc and/or boron nitride, amide compounds, sorbitol derivatives, metal salt of phosphate ester, etc. are known, however these do not have enough effects despite a high price and thus the utility value is not good in practical terms.
The non-patent references 1 and 2 suggest another method for increasing the crystallinity degree and the crystallization rate of a polylactic acid by adding a nucleating agent and a plasticizer. The above method is to improve polymer chain mobility and thus to increase a crystallization rate, by adding a certain amount of a plasticizer (anti-plasticization), unlike a method comprised of adding an excess amount of a plasticizer for imparting flexibility to a polymer. More specifically, it significantly enhanced the crystallization rate of a polylactic acid by mixing polylactic acid, talc, and a polyethylene glycol, however its crystallization rate was not sufficiently fast enough. Further, even though a polyethylene glycol is used at the amount of 5 wt % or less, it has a problem that bleed-out to the surface occurs so much that injection molding cannot be easily carried out.
For reducing bleed-out of a polyethylene glycol to the surface, Korean patent laid-open publication No. 10-2012-0035729, U.S. Pat. No. 7,351,785, etc. suggest a method to produce a polylactic acid-polyethylene glycol copolymer by adding a polyethylene glycol during polymerization of a polylactic acid. However, when forming a copolymer through polymerization as in these methods, the reaction time is long; an organic solvent that is harmful to a human body should be used at a great amount; and the reaction conditions should be carefully and finely adjusted cumbersomely. Further, these methods do not mention any of the crystallinity degree and the crystallization rate at all, and their object is to impart flexibility in the preparation of a polylactic acid film, and these have a problem that the crystallinity degree is lowered owing to the use of a high content of polyethylene glycol.